Radiographic images such as X-ray images have been widely utilized for diagnoses of a patient at medical scenes. A radiation imaging system using a radiation detector has come into widespread use in recent years. In this system, image data due to two-dimensional radiation is obtained as an electric signal by using a radiation detector, which is then processed and displayed on a monitor.
The scintillator panel serves to convert the radiation coming from the substrate side into light. The FPD (Flat Panel Detector) developed as an image radiographing apparatus in the 1990s is a radiation detector made up of a combination of a scintillator panel and image pickup element. In this case, cesium iodide (CsI) is often used as the material of the scintillator. The CsI has a relatively high conversion rate from X-ray to visible light and easily forms a columnar crystal structure by vacuum evaporation, and therefore, the scatter of the emitted light can be minimized by the light guiding effect.
However, the light emitting efficiency of the CsI alone is low. Accordingly, for example, a mixture of CsI and sodium iodide (NaI) blended at a desired mole ratio is deposited on a substrate via vacuum evaporation as sodium activated cesium iodide (CsI:Na), and annealing is provided in a post process, whereby the efficiency of conversion to visible light is enhanced. This is used as an X-ray phosphor (e.g., refer to Patent Document 1). Further, in recent years, a proposal has been made of an X-ray phosphor manufacturing method wherein an activator such as indium (In), thallium (Tl), lithium (Li), potassium (K), rubidium (Rb) and sodium (Ma) is formed by sputtering (e.g., refer to Patent Document 2).
However, the CsI-based scintillator is deliquescent and has a problem of deterioration with time. To avoid such degradation with time, a proposal has been made of a moisture proof protective layer formed on the surface of the CsI scintillator.
According to the conventional method, the sharpness of the image (hereafter also referred to as merely “sharpness”) obtained by the scintillator panel has been considered to increase as the thickness of the protective film becomes smaller. On the other hand, the thinner the thickness of the protective layer is, the lower the durability tends to be. To solve this problem, efforts have been made to select a protective film material that has functions to ensure durability while suppressing the deterioration in sharpness by minimizing the thickness of the protective layer. For example, attempts have been made to utilize a material having excellent strength and transparency such as polyethylene terephthalate, several types of resins in combination, or a protective layer having a multilayer structure.
However, in order to obtain long time conveyance durability, it has been necessary to increase the thickness of the protective film, whereby the sharpness has been lowered.
Patent Document 1 Examined Japanese Patent Publication No. 54-35060
Patent Document 2 Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2001-59899